Osteoporosis is a disease in which the density and quality of bone are reduced, leading to weakness of the skeleton and increased risk of fracture, particularly of the spine, wrist, hip, pelvis and upper arm. Osteoporosis and associated fractures are an important cause of mortality and morbidity. Bone quality is a crucial feature in osteoporosis and osteoblasts play a pivotal role as the bone forming cell and the director of bone resorption by osteoclasts. Osteoblasts have a meschenchymal origin and the differentiation of mesenchymal stem cells to an osteoblastic lineage is regulated by many endocrine, paracrine and autocrine factors. During bone formation osteoblasts produce an organic extracellular matrix (ECM) or osteoid (the immature matrix before mineralizing), which is composed primarily of type I collagen and non-collagenous proteins. This ECM then mineralizes, by the deposition thereon of calcium phosphates, to form bone spicules. Initiation of bone mineralization, or ossification, requires the precipitation and attachment of calcium phosphate crystals, in particular hydroxyapatite (Ca10(PO4)6(OH)2), to the ECM.
This process of mineralization is not unique for bone cells. It also occurs in hypertrophic chondrocytes as part of endochondral bone formation during growth. Furthermore, in pathological conditions of vascular calcification, mineralization of vascular smooth muscle cell (VSMC) ECM is believed to be involved in vascular calcification as well as in calcification of articular cartilage which occurs with ageing, degenerative joint diseases (e.g. osteoarthritis), and various metabolic and genetic disorders. Thus, ECM mineralization is a physiological process in bone and a pathological one in soft tissues.
Bone mineralization and in particular its regulation is a complicated process controlled by many factors including serum calcium and phosphate concentrations, hormones, enzymes, and the structure of the ECM. The macromolecular organization of type I collagen is a factor facilitating bone mineralization. Initially, calcium phosphate is deposited in the holes of the collagen fibrils and later fills in the pores and the rest of the space within the collagen fibrils.
Vascular calcification, and particular arterial calcification, has long been considered a passive process involving the necrotic core of the plaque serving as a nucleating centre for calcium phosphate mineral deposition. Recent scientific insights have challenged this hypothesis in that genetic aberrations of a variety of extracellular matrix (ECM) molecules, including matrix Gla protein and osteoprotegerin, result in spontaneous arterial calcification, indicating that like bone mineralization, arterial calcification is a delicately regulated process.
It is evident that the possibility of controlling the ECM mineralization process is desired, both in the direction of promoting mineralization as in the case of osteoporosis and fracture healing, and in the case of reducing or preventing mineralization as in the case of vascular calcification, in particular atherosclerotic calcification, and calcification of cartilage.
It is an aim of the present invention to provide methods for selecting candidate therapeutic agents for controlling mineralization of an extracellular matrix in a tissue of a subject, preferably in a mammal, most preferably a human. It is another aim of the present invention to provide methods for controlling ECM mineralization in soft tissue of a mammal, preferably a human, particularly in relation to pathological calcification of cartilage and blood vessels, such as mineralization of VSMC produced ECM and atherosclerosis. It is still another aim of the invention to provide methods of control by providing possibilities to stimulate mineralization on the one hand, and by providing possibilities to reduce or prevent mineralization on the other. It is a further aim to provide pharmaceutical compositions and medicaments for use in such methods.